1. Field of the Invention
The present invention relates to a zoom lens having a constant object-to-image distance for use in copying which has a brightness of the order of F.sub.NO 1:7 and which is capable of accommodating a viewing angle of up to 2.omega.=40.degree..
2. Background Art
With the increasing demand for smaller and less expensive copying machines, the need to reduce the size and cost of the lenses used in the copying machines is also growing. There is also a strong need to develop an improved version of the type of copying machine which is capable of magnification and reduction of the produced copy. Accordingly, the zoom lens for effecting the desired magnification and reduction must be improved. Prior art zoom lenses for use in copying are described in Unexamined Published Japanese patent application Nos. 68810/1982 and 57311/1985. The system shown in the first reference is composed of 8 lens elements, while the system of the second reference is composed of 7 lens elements, but neither system is completely satisfactory in terms of size and cost.
With a veiw to attaining further reduction in size, the present inventors previously filed Japanese patent applications No. 123991/1984 and No. 135731/1985 in which they proposed a zoom lens composed of 6 lens elements. These systems to some extent have met the requirements for reduction in size and cost but there still is room for improvement in terms of size and cost reduction since the number of lens elements, which account for a substantial portion of the cost of the system, has not been fully reduced. Also, the size is still too large since the distance between the lens groups, particularly in the system of No. 123991/1984, has to be changed by a large amount during zooming.